An amplifier may comprise a transconductance amplifier first stage, and an output stage, where the output stage provides unity voltage gain for driving a low impedance load. The output stage may comprise a source-follower transistor. Important criteria for an amplifier having a source-follower output stage include wide bandwidth, high current drive capability, low quiescent power consumption, high power supply rejection, and stable operation.
FIG. 1 illustrates in schematic diagram form, commonly used class AB amplifier 10. Class AB amplifier 10 includes amplifier 11 and output stage 12. Output stage 12 includes P-channel transistors 13, 14, and 15, current source 16, and capacitor 17. P-channel transistor 14 is coupled as a source-follower transistor for providing an output signal labeled "V.sub.OUT ". Amplifier 11 is a conventional single-stage transconductance amplifier for receiving a noninverting input signal labeled "+V.sub.IN ", and an inverting input signal labeled "-V.sub.IN ", and for providing a single-ended output signal.
Prior art class AB amplifier 10 provides low impedance output drive and strong current drive capability with relatively low quiescent power consumption. However, because of the large voltage gain and low frequency pole in output stage 12, Miller compensation capacitor 17 is required to stabilize amplifier 10. A problem with using Miller compensation is that it degrades the power supply rejection of output stage 12. In order for the drive current of output stage 12 to remain constant when the power supply varies, the gate-source voltage of transistor 13 should remain constant. For this to occur, the gate voltage of transistor 13 should track the power supply voltage. However, this causes the voltage across Miller compensation capacitor 17 to vary with the power supply voltage, and generates a power supply dependent current in capacitor 17 which degrades the amplifier power supply rejection.
Another problem with output stage 12 is that when required to drive a high voltage across a sufficiently small impedance load (not shown), the drain current in P-channel transistor 15 becomes larger than the current provided by current source 16. This causes the gate voltage of source-follower transistor 14 to rise. The current in source-follower transistor 14 is reduced to near zero as the gate voltage rises and source-follower transistor 14 becomes substantially non-conductive. This effectively disconnects the gain path in output stage 12 between the gate of P-channel transistor 15 and the output terminal of output stage 12. The voltage gain of output stage 12 reduces to that of a simple common-source transistor (P-channel transistor 13) in conjunction with the output load.